Editing Tidal locking

{{short description|Situation in which an astronomical object's orbital period matches its rotational period}}
[[File:Tidal locking of the Moon with the Earth.gif|thumb|300px|At left, the Moon rotates at the same rate it orbits the Earth, keeping the same face toward the planet. At right, if the Moon did not rotate then the face would change over the course of an orbit. Viewed from above; not to scale.]]
[[File:Pluto-Charon_system-new.gif|thumb|300px|A side view of the Pluto–Charon system. [[Pluto]] and [[Charon (moon)|Charon]] are tidally locked to each other.]]

'''Tidal locking''' between a pair of co-[[orbit]]ing [[astronomical body|astronomical bodies]] occurs when one of the objects reaches a state where there is no longer any net change in its [[rotation rate]] over the course of a complete orbit. In the case where a tidally locked body possesses synchronous rotation, the object takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the [[Moon]] always faces [[Earth]], although there is some [[libration|variability]] because the Moon's orbit is not perfectly circular. Usually, only the [[natural satellite|satellite]] is tidally locked to the larger body.<ref>{{cite web|title=When Will Earth Lock to the Moon?|url=http://www.universetoday.com/128350/will-earth-lock-moon/|website=Universe Today|date=2016-04-12|access-date=2017-01-02|archive-date=2016-09-23|archive-url=https://web.archive.org/web/20160923082215/http://www.universetoday.com/128350/will-earth-lock-moon/|url-status=live}}</ref> However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for [[Pluto]] and [[Charon (moon)|Charon]], and for [[Eris (dwarf planet)|Eris]] and [[Dysnomia (moon)|Dysnomia]]. Alternative names for the tidal locking process are '''gravitational locking''',<ref name=Clouse_et_al_2022/> '''captured rotation''', and '''spin–orbit locking'''. 

The effect arises between two bodies when their [[gravitational interaction]] slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates as a result of [[energy transfer|energy exchange]] and heat [[dissipation]]. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked.<ref name=Barnes_2010>{{cite book |title=Formation and Evolution of Exoplanets |editor1-first=Rory |editor1-last=Barnes |publisher=John Wiley & Sons |year=2010 |isbn=978-3527408962 |page=248 |url=https://books.google.com/books?id=-7KimFtJnIAC&pg=PA248 |access-date=2016-08-16 |archive-date=2023-08-06 |archive-url=https://web.archive.org/web/20230806163538/https://books.google.com/books?id=-7KimFtJnIAC&pg=PA248 |url-status=live }}</ref> The object tends to stay in this state because leaving it would require adding energy back into the system. The object's orbit may migrate over time so as to undo the tidal lock, for example, if a giant planet perturbs the object.

There is ambiguity in the use of the terms 'tidally locked' and 'tidal locking', in that some scientific sources use it to refer exclusively to 1:1 synchronous rotation (e.g. the Moon), while others include non-synchronous orbital resonances in which there is no further transfer of angular momentum over the course of one orbit (e.g. Mercury).<ref name=Heller_Leconte_Barnes_2011>{{cite journal |last1=Heller |first1=R. |last2=Leconte |first2=J. |last3=Barnes |first3=R. |title=Tidal obliquity evolution of potentially habitable planets |journal=Astronomy & Astrophysics |volume=528 |id=A27 |pages=16 |date=April 2011 |doi=10.1051/0004-6361/201015809 |bibcode=2011A&A...528A..27H |arxiv=1101.2156 |s2cid=118784209 }}</ref> In [[Mercury (planet)|Mercury's]] case, the planet completes three rotations for every two revolutions around the Sun, a 3:2 spin–orbit resonance. In the special case where an orbit is nearly circular and the body's rotation axis is not significantly tilted, such as the Moon, tidal locking results in the same hemisphere of the revolving object constantly facing its partner.<ref name=Barnes_2010/><ref name=Heller_Leconte_Barnes_2011/><ref>{{cite book |title=Mercury |first1=T. J. |last1=Mahoney |publisher=Springer Science & Business Media |year=2013 |isbn=978-1461479512 |url=https://books.google.com/books?id=iC65BAAAQBAJ&pg=PA44 |access-date=2018-04-20 |archive-date=2023-08-06 |archive-url=https://web.archive.org/web/20230806163607/https://books.google.com/books?id=iC65BAAAQBAJ&pg=PA44 |url-status=live }}</ref>
Regardless of which definition of tidal locking is used, the hemisphere that is visible changes slightly due to [[Libration|variations]] in the locked body's [[Orbital speed|orbital velocity]] and the [[Axial tilt|inclination of its rotation axis]] over time.

==Mechanism==
{{Further information|Centers of gravity in non-uniform fields}}

[[File:Árapály forgatónyomaték.png|thumbnail|Here, the body's tidal bulges (green) are misaligned with the direction of the attracting force (red). The local tidal forces (blue) exert a net torque that twists the body back toward realignment.]]

Consider a pair of co-orbiting objects, A and B. The change in [[Rotation period|rotation rate]] necessary to tidally lock body B to the larger body A is caused by the [[torque]] applied by A's [[gravity]] on bulges it has induced on B by [[tidal force]]s.<ref>{{cite book | title=Physics and Chemistry of the Solar System | first1=John | last1=Lewis | publisher=Academic Press | year=2012 | isbn=978-0323145848 | pages=242–243 | url=https://books.google.com/books?id=35uwarLgVLsC&pg=PA242 | access-date=2018-02-22 | archive-date=2023-08-06 | archive-url=https://web.archive.org/web/20230806163533/https://books.google.com/books?id=35uwarLgVLsC&pg=PA242 | url-status=live }}</ref>

The gravitational force from object A upon B will vary with distance, being greatest at the nearest surface to A and least at the most distant. This creates a gravitational [[gradient]] across object B that will distort its [[Mechanical equilibrium|equilibrium]] shape slightly. The body of object B will become elongated along the axis oriented toward A, and conversely, slightly reduced in dimension in directions [[orthogonal]] to this axis. The elongated distortions are known as [[tidal bulge]]s. (For the solid Earth, these bulges can reach displacements of up to around {{Convert|0.4|m|ftin|disp=or|abbr=on}}.<ref>{{cite journal | title=Impact of solid Earth tide models on GPS coordinate and tropospheric time series | display-authors=1 | last1=Watson | first1=C. | last2=Tregoning | first2=P. | last3=Coleman | first3=R. | journal=Geophysical Research Letters | volume=33 | issue=8 | pages=L08306 | date=April 2006 | doi=10.1029/2005GL025538 | bibcode=2006GeoRL..33.8306W | hdl=1885/21511 | url=http://eprints.utas.edu.au/3437/1/2005GL0255381.pdf | doi-access=free | access-date=2018-05-18 | archive-date=2021-11-26 | archive-url=https://web.archive.org/web/20211126171559/https://eprints.utas.edu.au/3437/1/2005GL0255381.pdf | url-status=live }}</ref>) When B is not yet tidally locked, the bulges travel over its surface due to orbital motions, with one of the two "high" tidal bulges traveling close to the point where body A is overhead. For large astronomical bodies that are nearly [[Sphericity|spherical]] due to self-gravitation, the tidal distortion produces a slightly [[prolate spheroid]], i.e. an axially symmetric [[ellipsoid]] that is elongated along its major axis. Smaller bodies also experience distortion, but this distortion is less regular.

The material of B exerts resistance to this periodic reshaping caused by the tidal force. In effect, some time is required to reshape B to the gravitational equilibrium shape, by which time the forming bulges have already been carried some distance away from the A–B axis by B's rotation. Seen from a vantage point in space, the points of maximum bulge extension are displaced from the axis oriented toward A. If B's rotation period is shorter than its orbital period, the bulges are carried forward of the axis oriented toward A in the direction of rotation, whereas if B's rotation period is longer, the bulges instead lag behind.

Because the bulges are now displaced from the A–B axis, A's gravitational pull on the mass in them exerts a torque on B. The torque on the A-facing bulge acts to bring B's rotation in line with its orbital period, whereas the "back" bulge, which faces away from A, acts in the opposite sense. However, the bulge on the A-facing side is closer to A than the back bulge by a distance of approximately B's diameter, and so experiences a slightly stronger gravitational force and torque. The net resulting torque from both bulges, then, is always in the direction that acts to synchronize B's rotation with its orbital period, leading eventually to tidal locking.

===Orbital changes===
[[File:tidal_acceleration_principle.svg|thumb|300px|In (1), a satellite orbits in the same direction as (but slower than) its parent body's rotation. The nearer tidal bulge (red) attracts the satellite more than the farther bulge (blue), slowing the parent's rotation while imparting a net positive force (dotted arrows showing forces resolved into their components) in the direction of orbit, lifting it into a higher orbit (tidal acceleration).<br/>In (2) with the rotation reversed, the net force opposes the satellite's direction of orbit, lowering it (tidal deceleration).]]
[[File:MoonTorque.svg|thumb|alt=Tidal Locking|If rotational frequency is larger than orbital frequency, a small torque counteracting the rotation arises, eventually locking the frequencies (situation depicted in green)]]
The [[angular momentum]] of the whole A–B system is conserved in this process, so that when B slows down and loses rotational angular momentum, its ''orbital'' angular momentum is boosted by a similar amount (there are also some smaller effects on A's rotation). This results in a raising of B's orbit about A in tandem with its rotational slowdown. For the other case where B starts off rotating too slowly, tidal locking both speeds up its rotation, and ''lowers'' its orbit.

===Locking of the larger body===
{{See also|Synchronous orbit}}
The tidal locking effect is also experienced by the larger body A, but at a slower rate because B's gravitational effect is weaker due to B's smaller mass. For example, Earth's rotation is gradually being slowed by the Moon, by an amount that becomes noticeable over geological time as revealed in the fossil record.<ref>{{cite book
 | first=Imke | last=de Pater
 | date=2001
 | title=Planetary Sciences
 | publisher=Cambridge| isbn=978-0521482196
 | page=34}}</ref> Current estimations are that this (together with the tidal influence of the Sun) has helped lengthen the Earth day from about 6 hours to the current 24 hours (over about 4.5 billion years). Currently, [[atomic clock]]s show that Earth's day lengthens, on average, by about 2.3 milliseconds per century.<ref>{{cite web|last = Ray|first = R.|date = 15 May 2001|url = http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html|archive-url = https://web.archive.org/web/20000818161603/http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html|archive-date = 18 August 2000|title = Ocean Tides and the Earth's Rotation|publisher = IERS Special Bureau for Tides|access-date =17 March 2010}}</ref> Given enough time, this would create a mutual tidal locking between Earth and the Moon. The length of Earth's [[day]] would increase and the length of a [[lunar month]] would also increase. Earth's sidereal day would eventually have the same length as the [[Orbit of the Moon|Moon's orbital period]], about 47 times the length of the Earth day at present. However, Earth is not expected to become tidally locked to the Moon before the Sun becomes a [[red giant]] and engulfs Earth and the Moon.<ref>{{cite book| last1 = Murray | first1 = C. D.|first2 = Stanley F. |last2 = Dermott| title = Solar System Dynamics| date = 1999| publisher = Cambridge University Press| isbn = 978-0-521-57295-8| page = 184 }}</ref><ref>{{cite book| last = Dickinson| first = Terence| author-link = Terence Dickinson| title = From the Big Bang to Planet X| date = 1993| publisher = [[Camden House]]| location = Camden East, Ontario| isbn = 978-0-921820-71-0| pages = 79–81 }}
</ref>

For bodies of similar size the effect may be of comparable size for both, and both may become tidally locked to each other on a much shorter timescale. An example is the [[dwarf planet]] [[Pluto]] and its satellite [[Charon (moon)|Charon]]. They have already reached a state where Charon is visible from only one hemisphere of Pluto and vice versa.<ref name=Michaely2017>{{citation | title=On the Existence of Regular and Irregular Outer Moons Orbiting the Pluto–Charon System | display-authors=1 | last1=Michaely | first1=Erez | last2=Perets | first2=Hagai B. | last3=Grishin | first3=Evgeni | journal=The Astrophysical Journal | volume=836 | issue=1 | id=27 | pages=7 | date=February 2017 | doi=10.3847/1538-4357/aa52b2 | bibcode=2017ApJ...836...27M | arxiv=1506.08818 | s2cid=118068933 | doi-access=free }}</ref>

===Eccentric orbits===
{{Quote
|text=A widely spread misapprehension is that a tidally locked body
permanently turns one side to its host.
|author=Heller et al. (2011)<ref name=Heller_Leconte_Barnes_2011/>
}}

For orbits that do not have an eccentricity close to zero, the [[rotation]] rate tends to become locked with the [[orbital speed]] when the body is at [[periapsis]], which is the point of strongest tidal interaction between the two objects. If the orbiting object has a companion, this third body can cause the rotation rate of the parent object to vary in an oscillatory manner. This interaction can also drive an increase in orbital eccentricity of the orbiting object around the primary &ndash; an effect known as eccentricity pumping.<ref name=Correia2012>{{citation
 | title=Pumping the Eccentricity of Exoplanets by Tidal Effect
 | last1=Correia | first1=Alexandre C. M. | last2=Boué | first2=Gwenaël
 | last3=Laskar | first3=Jacques | postscript=.
 | journal=The Astrophysical Journal Letters
 | volume=744 | issue=2 | id=L23 | pages=5 | date=January 2012
 | doi=10.1088/2041-8205/744/2/L23 | bibcode=2012ApJ...744L..23C | arxiv=1111.5486| s2cid=118695308 }}</ref>

In some cases where the orbit is [[eccentricity (orbit)|eccentric]] and the tidal effect is relatively weak, the smaller body may end up in a so-called '''spin–orbit resonance''', rather than being tidally locked. Here, the ratio of the rotation period of a body to its own orbital period is some simple fraction different from 1:1. A well known case is the rotation of [[Mercury (planet)|Mercury]], which is locked to its own orbit around the Sun in a 3:2 resonance.<ref name=Clouse_et_al_2022>{{citation |title=Spin-orbit gravitational locking-an effective potential approach |display-authors=1 |last1=Clouse |first1=Christopher |last2=Ferroglia |first2=Andrea |last3=Fiolhais |first3=Miguel C. N. |journal=European Journal of Physics |postscript= |volume=43 |issue=3 |id=035602 |pages=13 |date=May 2022 |doi=10.1088/1361-6404/ac5638 |arxiv=2203.09297 |bibcode=2022EJPh...43c5602C
|s2cid=246962304 }}</ref> This results in the rotation speed roughly matching the orbital speed around perihelion.<ref>{{citation |title=Rotational Period of the Planet Mercury |last=Colombo |first=G. |journal=Nature |volume=208 |issue=5010 |page=575 |date=November 1965 |doi=10.1038/208575a0 |bibcode=1965Natur.208..575C
|s2cid=4213296 |doi-access=free }}</ref>

Many [[exoplanet]]s (especially the close-in ones) are expected to be in spin–orbit resonances higher than 1:1. A Mercury-like terrestrial planet can, for example, become captured in a 3:2, 2:1, or 5:2 spin–orbit resonance, with the probability of each being dependent on the orbital eccentricity.<ref name=Makarov2012>{{citation |title=Conditions of Passage and Entrapment of Terrestrial Planets in Spin–orbit Resonances |last1=Makarov |first1=Valeri V. |journal=The Astrophysical Journal |volume=752 |issue=1 |id=73 |pages=8 |date=June 2012 |doi=10.1088/0004-637X/752/1/73 |bibcode=2012ApJ...752...73M  |arxiv=1110.2658 |s2cid=119227632 |postscript= }}</ref>

==Occurrence==

===Moons===
[[File:Synchronous rotation.svg|thumb|Due to tidal locking, the inhabitants of the central body will never be able to see the satellite's green area.]]
All twenty known moons in the [[Solar System]] that are [[List of gravitationally rounded objects of the Solar System|large enough to be round]] are tidally locked with their primaries, because they orbit very closely and tidal force increases rapidly (as a [[cubic function]]) with decreasing distance.<ref>{{cite book|last1=Schutz|first1=Bernard|title=Gravity from the Ground Up|publisher=Cambridge University Press|isbn=9780521455060|page=43|url=https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA43|access-date=24 April 2017|date=2003-12-04|archive-date=2023-08-06|archive-url=https://web.archive.org/web/20230806164032/https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA43|url-status=live}}</ref> On the other hand, most of the [[irregular satellite|irregular outer satellites]] of the [[giant planet]]s (e.g. [[Phoebe (moon)|Phoebe]]), which orbit much farther away than the large well-known moons, are not tidally locked.{{cn|date=May 2024}}

[[Pluto]] and [[Charon (moon)|Charon]] are an extreme example of a tidal lock. Charon is a relatively large moon in comparison to its primary and also has a very close [[orbit]]. This results in Pluto and Charon being mutually tidally locked. Pluto's other moons are not tidally locked; [[Styx (moon)|Styx]], [[Nix (moon)|Nix]], [[Kerberos (moon)|Kerberos]], and [[Hydra (moon)|Hydra]] all rotate [[chaos theory|chaotically]] due to the influence of Charon.<ref>{{cite journal | title=Resonant interactions and chaotic rotation of Pluto's small moons | last1=Showalter | first1=M. R. | last2=Hamilton | first2=D. P. | journal=Nature | date=June 2015 | volume=522 | issue=7554 | pages=45–49 | doi=10.1038/nature14469 | pmid=26040889 | bibcode=2015Natur.522...45S | s2cid=205243819 | url=https://esahubble.org/static/archives/releases/science_papers/heic1512a.pdf | access-date=2022-03-25 | archive-date=2022-06-08 | archive-url=https://web.archive.org/web/20220608040417/https://esahubble.org/static/archives/releases/science_papers/heic1512a.pdf | url-status=live }}</ref> Similarly, {{dp|Eris}} and [[Dysnomia (moon)|Dysnomia]] are mutually tidally locked.<ref name=Szakats2022/> {{dp|Orcus}} and [[Vanth (moon)|Vanth]] might also be mutually tidally locked, but the data is not conclusive.<ref name="Ortiz2011">{{Cite journal | last1 = Ortiz | first1 = J. L. | last2 = Cikota | first2 = A. | last3 = Cikota | first3 = S. | last4 = Hestroffer | first4 = D. | last5 = Thirouin | first5 = A. | last6 = Morales | first6 = N. | last7 = Duffard | first7 = R. | last8 = Gil-Hutton | first8 = R. | last9 = Santos-Sanz | first9 = P. | last10 = De La Cueva | first10 = I. | title = A mid-term astrometric and photometric study of trans-Neptunian object (90482) Orcus | doi = 10.1051/0004-6361/201015309 | journal = Astronomy & Astrophysics | volume = 525 | pages = A31 | date = 2010 |bibcode = 2011A&A...525A..31O |arxiv = 1010.6187 | s2cid = 56051949 }}</ref>

The tidal locking situation for [[asteroid moon]]s is largely unknown, but closely orbiting binaries are expected to be tidally locked,{{cn|date=March 2022|reason=Not necessarily true given the YORP effect}} as well as [[Contact binary (asteroid)|contact binaries]].

====Earth's Moon====
[[File:Lunation animation April 2007.gif|thumb|This simulation shows the variability in the portion of the Moon visible from Earth due to libration over the course of an orbit. Lighting phases from the Sun are not included.]]
Earth's Moon's rotation and orbital periods are tidally locked with each other, so no matter when the Moon is observed from Earth, the same hemisphere of the Moon is always seen. Most of the [[Far side (Moon)|far side of the Moon]] was not seen until 1959, when photographs of most of the far side were transmitted from the [[Soviet Union|Soviet]] spacecraft ''[[Luna 3]]''.<ref>{{cite web|title=Oct. 7, 1959 – Our First Look at the Far Side of the Moon|url=http://www.universetoday.com/105326/oct-7-1959-our-first-look-at-the-far-side-of-the-moon/|website=Universe Today|date=2013-10-07|access-date=2015-02-15|archive-date=2022-08-12|archive-url=https://web.archive.org/web/20220812035122/https://www.universetoday.com/105326/oct-7-1959-our-first-look-at-the-far-side-of-the-moon/|url-status=live}}</ref>

When Earth is observed from the Moon, Earth does not appear to move across the sky. It remains in the same place while showing nearly all its surface as it rotates on its axis.<ref>{{Cite web|last=Cain|first=Fraser|date=2016-04-11|title=When Will Earth Lock to the Moon?|url=https://www.universetoday.com/128350/will-earth-lock-moon/|access-date=2020-08-03|website=Universe Today|language=en-US|archive-date=2022-05-28|archive-url=https://web.archive.org/web/20220528015905/https://www.universetoday.com/128350/will-earth-lock-moon/|url-status=live}}</ref>

Despite the Moon's rotational and orbital periods being exactly locked, about 59 percent of the Moon's total surface may be seen with repeated observations from Earth, due to the phenomena of [[libration]] and [[parallax]]. Librations are primarily caused by the Moon's varying orbital speed due to the [[eccentricity (orbit)|eccentricity]] of its orbit: this allows up to about 6° more along its perimeter to be seen from Earth. Parallax is a geometric effect: at the surface of Earth observers are offset from the line through the centers of Earth and Moon; this accounts for about a 1° difference in the Moon's surface which can be seen around the sides of the Moon when comparing observations made during moonrise and moonset.<ref>{{cite book
 | title=The Moon and How to Observe It
 | first=Peter
 | last=Grego
 | year=2006
 | pages=47–50
 | isbn=9781846282430
 | publisher=Springer London
 | url=https://books.google.com/books?id=0wh5HABxVksC&pg=PA48
 | access-date=2023-03-19
 | archive-date=2023-10-21
 | archive-url=https://web.archive.org/web/20231021104054/https://books.google.com/books?id=0wh5HABxVksC&pg=PA48
 | url-status=live
 }}</ref>

===Planets===
It was thought for some time that [[Mercury (planet)|Mercury]] was in synchronous rotation with the Sun. This was because whenever Mercury was best placed for observation, the same side faced inward. Radar observations in 1965 demonstrated instead that Mercury has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun, which results in the same positioning at those observation points. Modeling has demonstrated that Mercury was captured into the 3:2 spin–orbit state very early in its history, probably within 10–20 million years after its formation.<ref name=Noyelles2012>{{Cite journal | bibcode=2014Icar..241...26N | last1=Noyelles | first1=Benoit | last2=Frouard | first2=Julien | last3=Makarov | first3=Valeri V. | last4=Efroimsky | first4=Michael | name-list-style=amp | title=Spin–orbit evolution of Mercury revisited | journal=Icarus | date=2014 | volume=241 | doi=10.1016/j.icarus.2014.05.045 | pages=26–44 | arxiv = 1307.0136 | s2cid=53690707 }}</ref>

The 583.92-day interval between successive close approaches of [[Venus]] to Earth is equal to 5.001444 Venusian solar days, making approximately the same face visible from Earth at each close approach. Whether this relationship arose by chance or is the result of some kind of tidal locking with Earth is unknown.<ref>{{cite journal |last1=Gold |first1=T. |last2=Soter |first2=S. |date=1969 |title=Atmospheric tides and the resonant rotation of Venus |journal=Icarus |volume=11 |issue=3 |pages=356–366|bibcode=1969Icar...11..356G |doi=10.1016/0019-1035(69)90068-2 }}</ref>

The [[exoplanet]] [[Proxima Centauri b]] discovered in 2016 which orbits around [[Proxima Centauri]], is almost certainly tidally locked, expressing either synchronized rotation or a 3:2 spin–orbit resonance like that of Mercury.<ref>{{cite journal|title=Tidal locking of habitable exoplanets|url=https://link.springer.com/article/10.1007/s10569-017-9783-7|publisher=Springer|year=2017|doi=10.1007/s10569-017-9783-7|last1=Barnes|first1=Rory|journal=Celestial Mechanics and Dynamical Astronomy|volume=129|issue=4|pages=509–536|s2cid=119384474|arxiv=1708.02981|bibcode=2017CeMDA.129..509B|access-date=2021-03-29|archive-date=2021-02-26|archive-url=https://web.archive.org/web/20210226135913/https://link.springer.com/article/10.1007/s10569-017-9783-7|url-status=live}}</ref>

One form of hypothetical tidally locked [[exoplanet]]s are [[eyeball planet]]s, which in turn are divided into "hot" and "cold" eyeball planets.<ref>{{cite web|url=http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets|title=Forget "Earth-Like"—We'll First Find Aliens on Eyeball Planets|publisher=Nautilus|language=en|author=Sean Raymond|date=20 February 2015|access-date=5 June 2017|archive-date=23 June 2017|archive-url=https://web.archive.org/web/20170623082602/http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets|url-status=dead}}</ref><ref name="SA-20200105">{{cite news |last=Starr |first=Michelle |title=Eyeball Planets Might Exist, And They're as Creepy as They Sound |url=https://www.sciencealert.com/eyeball-planets-might-exist-yep-they-re-exactly-as-creepy-as-they-sound |date=5 January 2020 |work=ScienceAlert.com |access-date=6 January 2020 |archive-date=6 January 2020 |archive-url=https://web.archive.org/web/20200106014046/https://www.sciencealert.com/eyeball-planets-might-exist-yep-they-re-exactly-as-creepy-as-they-sound |url-status=live }}</ref>

===Stars===
Close [[binary star]]s throughout the universe are expected to be tidally locked with each other, and [[extrasolar planet]]s that have been found to orbit their primaries extremely closely are also thought to be tidally locked to them. An unusual example, confirmed by [[MOST (satellite)|MOST]], may be [[Tau Boötis]], a star that is probably tidally locked by its planet [[Tau Boötis b]].<ref name="space.com">{{cite web |url=http://www.space.com/scienceastronomy/050523_star_tide.html |title=Role Reversal: Planet Controls a Star |date=2005-05-23 |first=Michael |last=Schirber |publisher=space.com |access-date=2018-04-21 |archive-date=2008-08-04 |archive-url=https://web.archive.org/web/20080804180104/http://www.space.com/scienceastronomy/050523_star_tide.html |url-status=live }}</ref> If so, the tidal locking is almost certainly mutual.<ref>{{cite journal | title=Life on a tidally-locked planet | last=Singal | first=Ashok K. | journal=Planex Newsletter | volume=4 | issue=2 | page=8 | date=May 2014 | bibcode=2014arXiv1405.1025S | arxiv=1405.1025 }}</ref><ref>{{cite journal | title=MOST detects variability on tau Bootis possibly induced by its planetary companion | url=http://www.aanda.org/articles/aa/full/2008/17/aa8952-07/aa8952-07.html | last1=Walker | first1=G. A. H. | last2=Croll | first2=B. | last3=Matthews | first3=J. M. | last4=Kuschnig | first4=R. | last5=Huber | first5=D. | last6=Weiss | first6=W. W. | last7=Shkolnik | first7=E. | last8=Rucinski | first8=S. M. | last9=Guenther | first9=D. B. | display-authors=1 | year=2008 | journal=Astronomy and Astrophysics | volume=482 | issue=2 | pages=691–697 | doi=10.1051/0004-6361:20078952 | arxiv=0802.2732 | bibcode=2008A&A...482..691W | s2cid=56317105 | access-date=2019-05-16 | archive-date=2021-02-25 | archive-url=https://web.archive.org/web/20210225212508/https://www.aanda.org/articles/aa/full/2008/17/aa8952-07/aa8952-07.html | url-status=live }}</ref>

==Timescale==
An estimate of the time for a body to become tidally locked can be obtained using the following formula:<ref>{{cite journal | author= B. Gladman| display-authors= etal| title= ''Synchronous Locking of Tidally Evolving Satellites''| journal= Icarus| date= 1996| volume= 122| issue= 1| pages= 166–192 | doi = 10.1006/icar.1996.0117| bibcode=1996Icar..122..166G| doi-access= free}} (See pages 169–170 of this article. Formula (9) is quoted here, which comes from S. J. Peale, ''Rotation histories of the natural satellites'', in {{cite book | editor= J. A. Burns | title= ''Planetary Satellites''| date= 1977| publisher= University of Arizona Press |pages= 87–112| location= Tucson}})</ref>

:<math>
t_{\text{lock}} \approx \frac{\omega a^6 I Q}{3 G m_p^2 k_2 R^5}
</math>

where
* <math>\omega\,</math> is the initial spin rate expressed in [[radian]]s [[Radian per second|per second]],
* <math>a\,</math> is the [[semi-major axis]] of the motion of the satellite around the planet (given by the average of the [[periapsis]] and [[apoapsis]] distances),
* <math>I\,</math> <math>\approx 0.4\; m_s R^2</math> is the [[moment of inertia]] of the satellite, where <math>m_s</math> is the mass of the satellite and <math>R</math> is the [[mean radius]] of the satellite,
* <math>Q\,</math> is the [[Rayleigh dissipation function|dissipation function]] of the satellite,
* <math>G\,</math> is the [[gravitational constant]],
* <math>m_p\,</math> is the mass of the planet (i.e., the object being orbited), and
* <math>k_2\,</math> is the tidal [[Love number]] of the satellite.

<math>Q</math> and <math>k_2</math> are generally very poorly known except for the Moon, which has <math>k_2/Q=0.0011</math>. For a really rough estimate it is common to take <math>Q \approx 100</math> (perhaps conservatively, giving overestimated locking times), and
:<math>
k_2 \approx \frac{1.5}{1+\frac{19\mu}{2\rho g R}},
</math>
where
* <math>\rho\,</math> is the density of the satellite
* <math>g\approx Gm_s/R^2</math> is the surface gravity of the satellite
* <math>\mu\,</math> is the rigidity of the satellite. This can be roughly taken as 3{{e|10}}&nbsp;N/m<sup>2</sup> for rocky objects and 4{{e|9}}&nbsp;N/m<sup>2</sup> for icy ones.

Even knowing the size and density of the satellite leaves many parameters that must be estimated (especially ''&omega;'', ''Q'', and ''&mu;''), so that any calculated locking times obtained are expected to be inaccurate, even to factors of ten. Further, during the tidal locking phase the semi-major axis <math>a</math> may have been significantly different from that observed nowadays due to subsequent [[tidal acceleration]], and the locking time is extremely sensitive to this value.

Because the uncertainty is so high, the above formulas can be simplified to give a somewhat less cumbersome one. By assuming that the satellite is spherical, <math>k_2\ll1\, , Q = 100</math>, and it is sensible to guess one revolution every 12 hours in the initial non-locked state (most asteroids have rotational periods between about 2 hours and about 2 days)

:<math>
t_{\text{lock}} \approx 6\ \frac{a^6R\mu}{m_sm_p^2} \times 10^{10}\ \text{years},
</math><ref>{{cite book
 | title=Planetary Habitability And Stellar Activity
 | first=Arnold
 | last=Hanslmeier
 | date=2018
 | page=99
 | isbn=9789813237445
 | publisher=World Scientific Publishing Company
 | url=https://books.google.com/books?id=plZoDwAAQBAJ&pg=PA99
 | access-date=2023-03-19
 | archive-date=2023-10-04
 | archive-url=https://web.archive.org/web/20231004190153/https://books.google.com/books?id=plZoDwAAQBAJ&pg=PA99
 | url-status=live
 }}</ref>

with masses in kilograms, distances in meters, and <math>\mu</math> in newtons per meter squared; <math>\mu</math> can be roughly taken as 3{{e|10}}&nbsp;N/m<sup>2</sup> for rocky objects and 4{{e|9}}&nbsp;N/m<sup>2</sup> for icy ones.

There is an extremely strong dependence on semi-major axis <math>a</math>.

For the locking of a primary body to its satellite as in the case of Pluto, the satellite and primary body parameters can be swapped.

One conclusion is that, ''other things being equal'' (such as <math>Q</math> and <math>\mu</math>), a large moon will lock faster than a smaller moon at the same orbital distance from the planet because <math>m_s\,</math> grows as the cube of the satellite radius <math>R</math>. A possible example of this is in the Saturn system, where [[Hyperion (moon)|Hyperion]] is not tidally locked, whereas the larger [[Iapetus (moon)|Iapetus]], which orbits at a greater distance, is. However, this is not clear cut because Hyperion also experiences strong driving from the nearby [[Titan (moon)|Titan]], which forces its rotation to be chaotic.

The above formulae for the timescale of locking may be off by orders of magnitude, because they ignore the frequency dependence of <math>k_2/Q</math>. More importantly, they may be inapplicable to viscous binaries (double stars, or double asteroids that are rubble), because the spin–orbit dynamics of such bodies is defined mainly by their viscosity, not rigidity.<ref>{{Cite journal|bibcode=2015AJ....150...98E |author=Efroimsky, M.  |title=Tidal Evolution of Asteroidal Binaries. Ruled by Viscosity. Ignorant of Rigidity. |journal=The Astronomical Journal |id=98 |date=2015 | volume=150 |issue=4 |doi=10.1088/0004-6256/150/4/98 |arxiv = 1506.09157 | pages=12 |s2cid=119283628 }}</ref>

==List of known tidally locked bodies==

===Solar System===
All the bodies below are tidally locked, and all but Mercury are moreover in synchronous rotation. (Mercury is tidally locked, but not in synchronous rotation.)

{| class="wikitable"
!style="white-space:nowrap;"| Parent body
! Tidally-locked satellites<ref>{{citation
 | title=Secular effects of tidal friction on the planet–satellite systems of the solar system
 | last1=Nobili | first1=A. M. | postscript=.
 | journal=Moon and the Planets
 | volume=18 | issue=2 | pages=203–216 | date=April 1978
 | doi=10.1007/BF00896743 | bibcode=1978M&P....18..203N | s2cid=121510792 }} "The following satellites seem to corotate: Phobos and Deimos, Amalthea, Io, Europa, Ganymede, Callisto, Janus, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Japetus, Miranda, Ariel, Umbriel, Titania, and Oberon."</ref>
|-
! [[Sun]]
| [[Mercury (planet)|Mercury]]<!-- Note: planet is tidally-locked in non-synchronized rotation, per the attached refs--><ref>{{citation
 | title=The rotational dynamics of Mercury and the state of its core
 | last1=Peale | first1=S. J.
 | journal=Mercury | publisher=University of Arizona Press | pages=461–493
 | year=1988 | bibcode=1988merc.book..461P | postscript=.
}}</ref><ref>{{citation
 | title=Past and present tidal dissipation in Mercury
 | pages=671 | display-authors=1 | last1=Rivoldini | first1=A.
 | last2=Beuthe | first2=M. | last3=van Hoolst | first3=T.
 | journal=European Planetary Science Congress 2010
 | date=September 2010 | bibcode=2010epsc.conf..671R
 | postscript=. }}</ref><ref name=Noyelles2012/> (3:2 spin–orbit resonance)
|-
! [[Earth]]
| [[Moon]]<ref>{{Cite web |title=The Moon's Orbit and Rotation |url=https://moon.nasa.gov/resources/429/the-moons-orbit-and-rotation |access-date=2023-08-24 |website=Moon: NASA Science |archive-date=2023-08-01 |archive-url=https://web.archive.org/web/20230801012352/https://moon.nasa.gov/resources/429/the-moons-orbit-and-rotation/ |url-status=live }}</ref>
|-
! [[Mars]]
| [[Phobos (moon)|Phobos]]<ref name=Correia2009/> · [[Deimos (moon)|Deimos]]<ref>{{citation
 | title=The dynamical evolution and origin of the Martian moons
 | last1=Burns | first1=J. A. | journal=Vistas in Astronomy
 | volume=22 | issue=2 | pages=193–208 | year=1978
 | doi=10.1016/0083-6656(78)90015-6 | bibcode=1978VA.....22..193B
 | postscript=. }}</ref>
|-
! [[Jupiter]]
| [[Metis (moon)|Metis]]<ref name=Burns_et_al_2004>{{citation | display-authors=1 | last1=Burns | first1=Joseph A. | last2=Simonelli | first2=Damon P. | last3=Showalter | first3=Mark R. | last4=Hamilton | first4=Douglas P. | last5=Porco | first5=Carolyn C. | last6=Throop | first6=Henry | last7=Esposito | first7=Larry W. | year=2004 | pages=241–262 | title=Jupiter's Ring-Moon System | work=Jupiter: The Planet, Satellites and Magnetosphere | publisher=Cambridge University Press | editor1-last=Bagenal | editor1-first=Fran | editor2-last=Dowling | editor2-first=Timothy E. | editor3-last=McKinnon | editor3-first=William B. | url=http://www.astro.umd.edu/~hamilton/research/preprints/BurSimSho03.pdf | access-date=2021-05-07 | bibcode=2004jpsm.book..241B | isbn=978-0-521-81808-7 | archive-date=2006-05-12 | archive-url=https://web.archive.org/web/20060512204155/http://www.astro.umd.edu/~hamilton/research/preprints/BurSimSho03.pdf | url-status=live }}</ref> · [[Adrastea (moon)|Adrastea]] · [[Amalthea (moon)|Amalthea]]<ref name=Burns_et_al_2004/> · [[Thebe (moon)|Thebe]]<ref name=Burns_et_al_2004/> · [[Io (moon)|Io]] · [[Europa (moon)|Europa]] · [[Ganymede (moon)|Ganymede]] · [[Callisto (moon)|Callisto]]
|-
! [[Saturn]]
| [[Pan (moon)|Pan]] · [[Atlas (moon)|Atlas]] · [[Prometheus (moon)|Prometheus]] · [[Pandora (moon)|Pandora]] · [[Epimetheus (moon)|Epimetheus]] · [[Janus (moon)|Janus]] · [[Mimas (moon)|Mimas]] · [[Enceladus (moon)|Enceladus]]<ref name=Dougherty_Spilker_2018>{{citation
 | title=Review of Saturn's icy moons following the Cassini mission
 | last1=Dougherty | first1=Michele K. | last2=Spilker | first2=Linda J.
 | journal=Reports on Progress in Physics
 | volume=81 | issue=6 | id=065901 | date=June 2018
 | page=065901 | doi=10.1088/1361-6633/aabdfb | pmid=29651989 | bibcode=2018RPPh...81f5901D | hdl=10044/1/63567 | s2cid=4810803 | hdl-access=free }}</ref> · [[Telesto (moon)|Telesto]] · [[Tethys (moon)|Tethys]]<ref name=Dougherty_Spilker_2018/> · [[Calypso (moon)|Calypso]] · [[Dione (moon)|Dione]]<ref name=Dougherty_Spilker_2018/> · [[Rhea (moon)|Rhea]]<ref name=Dougherty_Spilker_2018/> · [[Titan (moon)|Titan]] · [[Iapetus (moon)|Iapetus]]<ref name=Dougherty_Spilker_2018/>
|-
! [[Uranus]]
| [[Miranda (moon)|Miranda]] · [[Ariel (moon)|Ariel]] · [[Umbriel]] · [[Titania (moon)|Titania]] · [[Oberon (moon)|Oberon]]<ref>
{{citation
 | title=Red material on the large moons of Uranus: Dust from the irregular satellites?
 | last1=Cartwright | first1=Richard J. | last2=Emery | first2=Joshua P.
 | last3=Pinilla-Alonso | first3=Noemi | last4=Lucas | first4=Michael P.
 | last5=Rivkin | first5=Andy S. | last6=Trilling | first6=David E.
 | display-authors=1 | journal=Icarus
 | volume=314 | pages=210–231 | date=November 2018
 | doi=10.1016/j.icarus.2018.06.004 | arxiv=1806.01809
 | bibcode=2018Icar..314..210C 
| s2cid=119243937 }}</ref>
|-
! [[Neptune]]
| [[Proteus (moon)|Proteus]]<ref>{{citation
 | title=The Surfaces of Larissa and Proteus
 | last=Stooke | first=Philip J.
 | journal=Earth, Moon, and Planets
 | volume=65 | issue=1 | pages=3–54 | date=January 1994
 | doi=10.1007/BF00572198 | bibcode=1994EM&P...65...31S
}}</ref> · [[Triton (moon)|Triton]]<ref name=Correia2009>{{citation
 | title=Secular Evolution of a Satellite by Tidal Effect: Application to Triton
 | last1=Correia | first1=Alexandre C. M.
 | journal=The Astrophysical Journal Letters
 | volume=704 | issue=1 | pages=L1–L4 | date=October 2009
 | doi=10.1088/0004-637X/704/1/L1 | bibcode=2009ApJ...704L...1C
 | arxiv=0909.4210 | s2cid=15378780 | postscript=. }}</ref>
|-
! [[Pluto]]
| [[Charon (moon)|Charon]] (mutually locked)<ref name=Michaely2017/>
|-
! [[Eris (dwarf planet)|Eris]]
| [[Dysnomia (moon)|Dysnomia]] (mutually locked)<ref name="Szakats2022">{{cite journal
  |display-authors = etal
  |first1 = R. |last1 = Szakáts
  |first2 = Cs. |last2 = Kiss
  |first3 = J. L. |last3 = Ortiz
  |first4 = N. |last4 = Morales
  |first5 = A. |last5 = Pál
  |first6 = T. G. |last6 = Müller
  |title      = Tidally locked rotation of the dwarf planet (136199) Eris discovered from long-term ground based and space photometry
  |journal    = Astronomy & Astrophysics
  |year       = 2023
  |volume = L3 |page = 669 |doi = 10.1051/0004-6361/202245234 |arxiv      = 2211.07987
|bibcode = 2023A&A...669L...3S |s2cid = 253522934 }}</ref>
|}

===Extra-solar===
* The most successful detection methods of exoplanets (transits and radial velocities) suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone, which makes it difficult to estimate the true incidence of this phenomenon.<ref>{{cite journal|author=F. J. Ballesteros|author2=A. Fernandez-Soto|author3=V. J. Martinez|title=Title: Diving into Exoplanets: Are Water Seas the Most Common?|date=2019|doi=10.1089/ast.2017.1720|journal=[[Astrobiology (journal)|Astrobiology]]|pmid=30789285|volume=19|issue=5|pages=642–654|hdl=10261/213115|s2cid=73498809|hdl-access=free}}</ref> [[Tau Boötis]] is known to be locked to the close-orbiting [[giant planet]] [[Tau Boötis b]].<ref name="space.com" />

==Bodies likely to be locked==

===Solar System===
Based on comparison between the likely time needed to lock a body to its primary, and the time it has been in its present orbit (comparable with the age of the Solar System for most planetary moons), a number of moons are thought to be locked. However their rotations are not known or not known enough. These are:

====Probably locked to Saturn====
{{colbegin|colwidth=18em}}
* [[Daphnis (moon)|Daphnis]]
* [[Aegaeon (moon)|Aegaeon]]
* [[Methone (moon)|Methone]]
* [[Anthe (moon)|Anthe]]
* [[Pallene (moon)|Pallene]]
* [[Helene (moon)|Helene]]
* [[Polydeuces (moon)|Polydeuces]]
{{colend}}

====Probably locked to Uranus====
{{colbegin|colwidth=18em}}
* [[Cordelia (moon)|Cordelia]]
* [[Ophelia (moon)|Ophelia]]
* [[Bianca (moon)|Bianca]]
* [[Cressida (moon)|Cressida]]
* [[Desdemona (moon)|Desdemona]]
* [[Juliet (moon)|Juliet]]
* [[Portia (moon)|Portia]]
* [[Rosalind (moon)|Rosalind]]
* [[Cupid (moon)|Cupid]]
* [[Belinda (moon)|Belinda]]
* [[Perdita (moon)|Perdita]]
* [[Puck (moon)|Puck]]
* [[Mab (moon)|Mab]]
{{colend}}

====Probably locked to Neptune====
{{colbegin|colwidth=18em}}
* [[Naiad (moon)|Naiad]]
* [[Thalassa (moon)|Thalassa]]
* [[Despina (moon)|Despina]]
* [[Galatea (moon)|Galatea]]
* [[Larissa (moon)|Larissa]]
{{colend}}

====Probably mutually tidally locked====
*[[90482 Orcus|Orcus]] and [[Vanth (moon)|Vanth]]<ref name="Brown2023">{{cite journal
  |first1 = Michael E. |last1 = Brown
  |first2 = Bryan |last2 = Butler
  |title      = Masses and densities of dwarf planet satellites measured with ALMA
  |journal    = The Planetary Science Journal
  |date       = July 2023
  |volume     = 4
  |issue      = 10
  |id         = 
  |pages      = 11
  |doi-access = free
  |doi        = 10.3847/PSJ/ace52a
  |arxiv      = 2307.04848
  |bibcode    = 2023PSJ.....4..193B
  |s2cid      = }}</ref>

===Extrasolar===
* [[Gliese 581c]],<ref>{{cite news | url=https://www.usatoday.com/printedition/news/20070425/1a_bottomstrip25_dom.art.htm | work=USA Today | title=Out of our world: Earthlike planet | first=Dan | last=Vergano | date=2007-04-25 | access-date=2010-05-25 | archive-date=2011-05-23 | archive-url=https://web.archive.org/web/20110523021921/http://www.usatoday.com/printedition/news/20070425/1a_bottomstrip25_dom.art.htm | url-status=live }}</ref> [[Gliese 581g]],<ref>{{cite journal|url=http://news.sciencemag.org/sciencenow/2010/09/astronomers-find-most-earth-like.html|title=Astronomers Find Most Earth-like Planet to Date|journal=Science, USA|date=September 29, 2010|access-date=September 30, 2010|archive-url=https://web.archive.org/web/20101002020745/http://news.sciencemag.org/sciencenow/2010/09/astronomers-find-most-earth-like.html|archive-date=October 2, 2010}}</ref><ref>{{Cite web|url=https://www.telegraph.co.uk/science/space/8033124/Gliese-581g-the-most-Earth-like-planet-yet-discovered.html|title=Gliese 581g the most Earth like planet yet discovered|publisher=[[The Daily Telegraph]], UK|date=September 30, 2010|access-date=September 30, 2010|archive-url=https://web.archive.org/web/20101002104629/http://www.telegraph.co.uk/science/space/8033124/Gliese-581g-the-most-Earth-like-planet-yet-discovered.html|archive-date=October 2, 2010}}</ref> [[Gliese 581b]],<ref>{{cite web |title=Gliese 581 |url=http://www.openexoplanetcatalogue.com/planet/Gliese%20581%20b/ |website=Open Exoplanet Catalogue |access-date=16 May 2019 |archive-date=7 April 2022 |archive-url=https://web.archive.org/web/20220407173703/http://www.openexoplanetcatalogue.com/planet/Gliese%20581%20b/ |url-status=live }}</ref> and [[Gliese 581e]]<ref>{{cite web |title=Gliese 581 |url=https://library.eb.com.au/levels/adults/article/Gliese-581/475108 |website=Encyclopedia Britannica |access-date=16 May 2019 |archive-date=6 August 2023 |archive-url=https://web.archive.org/web/20230806164036/https://library.eb.com.au/?target=%2Flevels%2Fadults%2Farticle%2FGliese-581%2F475108 |url-status=live }}</ref> may be tidally locked to their parent star [[Gliese 581]]. [[Gliese 581d]] is almost certainly captured either into the 2:1 or the 3:2 spin–orbit resonance with the same star.<ref>{{Cite journal | bibcode=2012ApJ...761...83M | last1=Makarov | first1=V. V. | last2=Berghea | first2=C. | last3=Efroimsky | first3=M. | name-list-style=amp | title=Dynamical Evolution and Spin–Orbit Resonances of Potentially Habitable Exoplanets: The Case of GJ 581d. | journal=The Astrophysical Journal | id=83 | date=2012 | issue=2 | volume=761 | doi=10.1088/0004-637X/761/2/83 | pages=83 | arxiv=1208.0814 | s2cid=926755 }}</ref>
* All planets in the [[TRAPPIST-1]] system are likely to be tidally locked.<ref>{{cite press release|title=NASA Telescope Reveals Largest Batch of Earth-Size, Habitable-Zone Planets Around Single Star|url=https://www.nasa.gov/press-release/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around|publisher=NASA|date=22 February 2017|access-date=23 February 2017|archive-date=5 March 2017|archive-url=https://web.archive.org/web/20170305055703/https://www.nasa.gov/press-release/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around/|url-status=live}}</ref><ref>{{Cite journal | last1=Gillon | first1=Michaël | last2=Triaud | first2=Amaury H. M. J. | last3=Demory | first3=Brice-Olivier | last4=Jehin | first4=Emmanuël | last5=Agol  | first5=Eric | last6=Deck | first6=Katherine M. | last7=Lederer | first7=Susan M. | last8=de Wit | first8=Julien | last9=Burdanov | first9=Artem | date=2017-02-23 | title=Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1 | journal=Nature | language=en | volume=542 | issue=7642 | pages=456–460 | doi=10.1038/nature21360 | issn=0028-0836 | pmc=5330437 | pmid=28230125 | arxiv=1703.01424 | bibcode=2017Natur.542..456G  }}</ref>

==See also==
{{Portal|Physics|Astronomy|Stars|Spaceflight|Outer space|Solar System}}
* {{annotated link|Conservation of angular momentum}}
* {{annotated link|Gravity-gradient stabilization}}
* {{annotated link|Kozai mechanism}}
* {{annotated link|Orbital resonance}}
* {{annotated link|Planetary habitability}}
* [[Pseudo-synchronous rotation]] – a near synchronization of revolution and rotation at [[periastron]]
* {{annotated link|Roche limit}}
* {{annotated link|Synchronous orbit}}
* {{annotated link|Tidal acceleration}}
* {{annotated link|Rotation around a fixed axis}}

==References==
{{Reflist|30em}}

{{The Moon|state=collapsed}}

{{DEFAULTSORT:Tidal Locking}}
[[Category:Celestial mechanics]]
[[Category:Orbits]]
[[Category:Tidal forces|Locking]]